Craniofacial Muscles

186 J.C. Stemple et al.
established during morphogenesis and is later regulated and in fl uenced by musclegroup
speci fi c patterns of gene expression (Noden and Francis-West 2006 ; Spencer
and Porter 2006 ; Cheng et al. 2004 ; Fischer et al. 2005 ) .
The consequences of the biological and functional diversity between craniofacial
and limb skeletal muscle are signi fi cant. Specialized phenotypes of craniofacial
muscles likely underlie and permit these muscles to (1) engage in extremely rapid
yet prolonged contraction, (2) contribute to highly re fi ned patterns of movement,
(3) recover consequently from mechanical and neurological insult, (4) resist the
in fl uence of aging and lastly, (5) escape the pathological cascade of select neuromuscular
diseases (Porter and Baker 1996 ; Zemlin 1988 ; Spencer and Porter 2006 ;
Cheng et al. 2004 ; Marques et al. 2007 ; McLoon et al. 2004, 2007 ; Muller et al.
2001 ; Norton et al. 2001 ; Thomas et al. 2008 ; Pavlath et al. 1998 ) . One subset of the
craniofacial muscles emerging as highly specialized in the mammal is the intrinsic
laryngeal muscles (ILMs). The ILMs are intricately involved in the life-sustaining
functions of respiration, airway protection, and swallowing, with critical secondary
functions in vocalization and communication behaviors (i.e., human speech).
11.1 Differences in Limb vs. Craniofacial Muscles
Point to the Existence of Unique Phenotypes
It has been well established that limb skeletal muscle has the capacity to regenerate
in the face of injury via the action of satellite cells. After myo fi ber injury for example,
satellite cells progress from a quiescent state to an active state. Once active,
these cells move to the site of injury, fuse with one another, and differentiate into
new myo fi bers (Mauro 1961 ) . However, recent work in the extraocular muscles
(EOM) and laryngeal muscles of rabbits suggests that myo fi ber remodeling is an
ongoing event in these select muscle groups, occurring in the absence of any apparent
fi ber injury (McLoon et al. 2004 ; Goding et al. 2005 ; Shinners et al. 2006 ) .
Seminal work in this area by McLoon et al. ( 2004 ) found evidence of continual
myonuclear removal and addition in uninjured single fi bers of rabbit EOM.
Remodeling was noted to proceed at a rate of one myonuclear addition per 1,000
myo fi bers in cross section every 12 h. Subsequent work by Goding et al. ( 2005 )
identi fi ed similar patterns of uninjured fi ber remodeling in rabbit thyroarytenoid
(TA) and posterior cricoarytenoid (PCA) muscles, estimating that myonuclear addition
in the laryngeal muscles occurred at a rate of 2 myonuclei per 1,000 myo fi bers
in cross section per 24 h. Together, these fi ndings suggested that muscle precursor
cells, generally quiescent in limb skeletal muscle, are continuously active in subsets
of craniofacial muscles, and that this enhanced remodeling capacity may be related
to the great potential of these muscles to recover after insult (Goding et al. 2005 ;
McLoon et al. 2004 ) .
Along this same vein, the laryngeal muscles have long been recognized for their
ability to survive and reinnervate following neurological insult (Gardner and
Benninger 2006 ; Nomoto et al. 1993 ; Shindo et al. 1992 ) . Following denervation of